The present invention relates to stencil tensioning frames and, more particularly, to a balanced stencil foil tensioning frame having a foil alignment fixture.
Current manufacturing techniques for making printed circuit boards call for the mounting of components to the circuit board by surface mounting. Rather than passing the leads of a component through holes in a circuit board and then soldering the leads at the holes, the circuit board is provided with pads of solder paste and the components are placed on the pads for soldering directly to the surface of the board.
In order to place the pads of paste in appropriate locations on the circuit board, just before affixing the components to the board, a stencil-like screen is juxtaposed with the circuit board and to provide a pattern of openings registered with the locations where the pads are to be placed. The paste then is spread over the screen with a squeegee to lay down the pattern of pads. The stencil screen is typically in the form of a thin foil which is supported by a stretcher frame in an appropriately tensioned state. The frame fits into a machine which registers the tensioned screen with the circuit board and then applies the paste. Typically, the machine also stretches the stencil to achieve the desired tension of the stencil to ensure accuracy during printing.
Conventional stencil tensioning frames typically use bladders and air to either tension or release a stencil foil therefrom. These systems are usually unreliable and heavy as, for example, the bladder used typically leaks or fails completely, requiring in-depth repairs on a frequent basis. Air pressure variation can cause mis-alignment of foils and therefore cumbersome operation. Moreover, these conventional systems use a complex arrangement of springs and pneumatics, resulting in even greater possibility of failure. These conventional stencil tensioning frames often require large, flat horizontal surfaces to aid in aligning the foils.
Other conventional systems use mesh mounted foils. In these systems, tension is provided by a stretched mesh and the foil is glued to this mesh.
As can be seen, there is a need for an improved stencil foil tensioning frame.